Php FORM (checkbox control)

To analyze this control we will use practically the same example as with the radio object:
Implement a form that requests the loading of two integers, one in each text. Have two checkbox-type controls that allow us to select whether we want to add and / or subtract the entered values.

If the checkbox is not selected in the form, an entry is not created in the associative vector $_REQUEST, to find out if a certain component exists in a vector, the isset function is used, if it returns true it means that it exists and therefore the checkbox is selected.
We have two if at the same height since the two controls of type checkbox could be selected.